1. Field of the Invention
This invention relates generally to rolling mills producing long products such as rods and bars, and is concerned in particular with the provision of an improved modular rolling mill.
2. Description of the Prior Art
Block-type rolling mills are known, as disclosed for example in U.S. Pat. Nos. 4,537,055; 5,152,165; 6,134,930 and 6,546,776. These serve primarily as finishing trains in rod rolling mills, and are in widespread use throughout the world. Block type mills comprise a series of roll stands arranged in sequence to roll an alternating oval/round pass sequence, with the roll shafts of the round roll stands being staggered by 90° with respect to the roll shafts of the oval roll stands in order to roll products in a twist free manner.
Typically, the roll stands having round and oval roll passes are staggered on opposite sides of the mill pass line and are mechanically coupled to respective parallel line shafts driven at different speeds by a common mill drive connected to the line shafts by a differential gear box. Other known drive arrangements for block type mills employ a single line shaft as disclosed for example in U.S. Pat. Nos. 3,587,277 and 6,161,412, and chain drives, as disclosed in U.S. Pat. No. 4,129,023.
Although block type mills are capable of rolling products at high delivery speeds, e.g., 5.5 mm rod at 120 m/sec., their overall efficiency is compromised by a lack of flexibility when changing from one product size to another.
For example, as shown in FIG. 4, in a typical rolling program for a ten stand block type mill, when all stands are operative, a feed size having a 17 mm diameter will be rolled into a finished product having a diameter of 5.5 mm. In order to roll a finished product having a 7.0 mm diameter, roll stands 9 and 10 are rendered inoperative (a procedure commonly referred to as “dummying”). Progressively larger product sizes can be rolled by progressively dummying additional roll stands, e.g., stands 7 and 8 to roll 8.5 mm rounds, stands 5 and 6 to roll 10.5 mm rounds, etc. Because the successive roll stands are integrally joined together, in order to effect dummying, the work rolls must be removed and replaced with guides. This is a labor intensive procedure, typically taking between 20–60 minutes to complete, and occurring every 8 hours. If, conservatively speaking, one assumes an average down time of 30 minutes for each dummying operation, with a mill operating 300 days per year at a rate of 60 tons/hr, the lost production can amount to upwards of 27,000 tons/year.
In order to achieve improved efficiencies, modular mills have been developed, as disclosed for example in U.S. Pat. Nos. 5,595,083 and 6,053,022. These mills employ multiple motors driving gear boxes detachably coupled to pairs of successive rolling units. The rolling units each include roll stands with oval and round roll passes, and are interchangeable and rapidly shiftable onto and off of the mill pass line to thereby accommodate the rolling of different product sizes. Although mechanically sound and advantageously flexible, as compared to block type mills, such modular arrangements are relatively complex and expensive, both to purchase and subsequently to maintain.
The objective of the present invention is to provide an improved modular rolling mill that overcomes or at least substantially mitigates the disadvantages associated with conventional modular and block type mills.